Cage Design Questions

#1

My car is finally going to be getting a cage next week. I would like everyone’s opinions and suggestions on the following issues.

Tubing size: I am leaning towards using the 1.5 x 0.120 vs. the 1.75 x 0.095. Is that a mistake? I was thinking the 1.5 for better visibility.

Door bars: I like the design that the photos below show. Any one have any comments or suggestions? I want the car to be legal for other clubs as well, so anyone have any experience with BMWCCA and SCCA? I would rather not have NASCAR bars if possible…



(Are the gussets in these, two bars vertically and plates welded to them, or are they bent sheet metal?)

Main Hoop: Should I have a single diagonal or a cross?

Foot Protection: Is it worth it? Has anyone seen someone in an e30 need this? (I guess I would rather be safer than sorry…)

Rear Mount: I am thinking of going back to the spring pickup point…Is that the preferred place, or is the shock tower better?

I am looking to make this a really safe cage, I would much rather be safer than sorry…Any general suggestions would be greatly appreciated!

#2

I would stick with the 1.75 X .095 as it is lighter. Not much difference in visibility. Those door bars are fine and are what some are using to meet the requirement of two solid bars in the door area. I used the standard X with one bar solid and one cut and welded to the other. I then used a solid bar across the bottom at the rocker area. If you want the safest use the X in the main hoop and make sure the bar from the drivers head to the passanger floor is the solid bar and all the others welded to it. I took the rear bars to the spring area. The mounting plate was welded to the floor and up the side of the shock tower. The bar is welded to the floor and up the tower a few inches.

When designing your cage look at the different areas you may get hit and see where the force is going to go. As an example if you just have one diagonal in the main hoop and you get hit on the passanger side down low there is no place for the force to be transmitted through the cage. If you have the second diagonal the force will be transfered through the diagonal. Look at all the corners and front and back using this idea.

I have foot protection FWIW and think it necessary.

Good Luck.

Michael

#3

I would say foot protection is a must.

#4

I have seen several wrecked E30s in the junkyard and footwell protection is essential. I’d consider that one of the weaker aspects of the chassis.

I pretty much agree with everything that Michael stated. We added the rocker panels bars to ours. Just be sure you don’t trap the wiring harness behind the cage. :ohmy:

#5

1.75" is 5% lighter than the smaller diameter due to thin wall.

If using 4130, a 2 foot long section would have:

1.75" (3.35 lbs), compressive strength: 32551 lbs, in Tension: 46924 lbs, Column buckling stress= 65900 psi
1.5" (3.53 lbs), compressive strength: 32097 lbs, in Tension: 49424 lbs, Column buckling stress= 61696 psi

If using 4130, a 4 foot long section would have:

1.75" (6.71 lbs), compressive strength: [color=#008000]20268[/color] lbs, in Tension: 46924 lbs, Column buckling stress= [color=#008000]41034[/color] psi
1.5" (7.07 lbs), compressive strength: [color=#FF0000]15489[/color] lbs, in Tension: 49424 lbs, Column buckling stress= [color=#FF0000]29773[/color] psi

The superior choice is, I believe, self evident when comparing the figures. 1.75" retains a safety margin over [color=#0000FF]longer[/color] spans. Like door bars. Like getting [color=#FF6600]rammed[/color].

Disclaimer: The above data is correct as far as I know, but use at your own risk. Calculations performed by Excel spreadsheet written by Neal Willford for Sport Aviation.

#6

interesting data Jamie.
If a door is being rammed, X bars will be in tension (when the two sizes are similar in performance). NASCAR bar style might be put in compression first and then tension.
cheers,
bruce

#7

can you guys with foot box protection show what you’ve built in that area?

#8

Here is a pic I just took of my old 635 race car to send to someone. I did basically the same thing in the Spec E30. Also, with my X door bars they are bent slightly outward for the reasons mentioned above.

Michael

#9

so by tieing the cage into the base of the firewall it provides foot well protection? I didn’t know if you guys were building pedal boxes like they do in rally.

#10

no on the pedal boxes, you’re allowed six points of attachment for the main cage, then an optional two attachment points going forward as Michael’s pic shows.
cheers,
bruce

Shawn Manny wrote:

#11

Here is the design I am copying…minus the rear brace between the rear mounts and the gusset on the front hoop. I am using the bent door bars also…Its not too late to change it. Please let me know if I have (due to the rules) to change anything, or if there is a stronger way to do something?

#12

Brenden Selvig wrote:

[quote] Please let me know if I have (due to the rules) to change anything, or if there is a stronger way to do something?

[/quote] My planned cage is of a very similar design however I addded a roof section diagonal from behind on the drivers side to forward on the passenger side… for my peace of mind…literally.

#13

I had to add a center roof bar as the diagonals interfered with head room. I’m 6’1" and long torso-ed.

That cage looks nearly identical to mine. Nice looking cage.

#14

Related question. Had my cage done last year - followed BMWCCA/NASA regulations. Now, of course, I need 2 bars on the driver’s side. I attached a picture of what I have. I love the cage (it is a Mark McMahon cage from Akron, OH - very good fabricator), but now that I am in Florida (Tampa), I don’t know how best to meet the rules requirement or who to go to.

I was thinking of another bar below the current bar, attaching about an inch below where the front of my door bar mounts now, and run it horizontally rearward to meet the upright about 6 inches from the bottom of the "hoop". I’d rather not do the traditional "X" design, as I’d rather have 2 solid bars than 1 solid and 2 split bars. I don’t want to pull the current bar and do the "X" with the 2 sideways "V"s that looks like a great design, but involves more re-do than I want. What do you guys think?

Cage_DoorBar.jpg680x510

#15

Like this…

Cage_DoorBar1.JPG680x510

#16

John - I had a similar situation as you with the #30 car. My additional bar is about the same as the one you have drawn in, but a little higher in the back and butted up to the front of the other bar, thinking that the b-pillar and the boxed-in section along the floor provide some impact protection, FWIW.
Ed

#17

Here’s the basic BeerTech cage…

#18

I think I would end up with the "BeerTech" cage, without the 2 smaller pieces that make the "X" (and the solid piece that makes your "X" would be going the other direction, not that this should matter). I think I will ask for 1 (or maybe 2) of those uprights between the 2 door bars.

Ed, I agree with the concept of moving it up…

Cage_DoorBar2.JPG680x510

#19

Brenden Selvig wrote:

[quote]Here is the design I am copying…minus the rear brace between the rear mounts and the gusset on the front hoop. I am using the bent door bars also…Its not too late to change it. Please let me know if I have (due to the rules) to change anything, or if there is a stronger way to do something?

[/quote]

Why thank you. ; ) We ended up modifying that design slightly for Simon’s cage…basically I think we only changed the door bar design. The NASA tech inspectors in our region love his cage.

#20

We ended up not putting the bars at the bottom of the door and moved the connection point for the rear bars.
After a year of racing with this setup… there is very little I would have done differently.